An image forming device forms images by transferring toner to a media sheet. The toner is initially stored within an image forming station and then transferred through a series of steps to ultimately be transferred to the media sheet. This process requires that one or more different operating parameters be set to allow the toner to move from the image forming station and ultimately reach the media sheet.
For a color image forming device, multiple image forming stations are included that each contain a different color toner. Each color toner is transferred from their respective image forming stations independently of the other color toners. At some point in the process, the different color toners are combined together to form the overall color image.
Different operating parameters may be used to transfer the different color toners. By way of example, a first set of parameters is used to transfer black toner to the media sheet, and a second set of parameters is used to transfer yellow toner. These parameters are set to ensure a proper amount of each toner is transferred to the media sheet. In the event not enough toner is transferred, the toner image on the media sheet may be too light relative to the other colors and result in a print defect. Likewise, too much transferred toner may cause the image to be too dark relative to the other colors.
Prior art devices determine the operating parameters by a toner patch sensing procedure. This procedure includes transferring a toner image for each color from the respective image forming station under a set of predetermined operating parameters. A sensor within the device then detects the transferred toner to monitor the toner density of unfused images and provide a means of controlling the print darkness. This information is then used to adjust laser power, photoconductor charge, developer bias, and other process conditions that affect image density. The toner patch sensing procedure is also used to maintain the color balance and in some cases to modify the gamma correction or halftone linearization as the electrophotographic process changes with the environment and aging effects.
A drawback of the prior art devices is the toner patch sensing procedure takes time to form each test toner image and sense the image. The testing procedure may reduce overall throughput of the device, particularly for the first sheet of a print job when the testing procedure is more likely to occur. Further, the test toner is discarded prior to actual image formation of the print job. This causes additional waste toner, and also requires that the waste toner be accommodated or somehow discarded.
Another drawback of toner patch sensing is the inability to accurately detect a black toner patch on a dark surface. This may occur if the black toner patch is placed on a black belt. Cyan, magenta, and yellow toners are each fairly reflective and can be accurately detected. However, the black toner does not have the same reflectivity which may prevent an accurate detection.